1. Field of the Invention
The present disclosure relates to methods for preparing and purifying electrolytes and electrolyte solutions useful in reduction-oxidation (redox) flow cell batteries.
2. Description of the Relevant Art
There is an increasing demand for novel and innovative electric power storage systems. Redox batteries have become an attractive means for such energy storage. In certain applications, a redox flow cell battery or a flow cell battery may include positive and negative electrodes disposed in separate half-cell compartments. The two-half cells may be separated by a porous or ion-selective membrane, through which ions are transferred during a redox reaction. Electrolytes (anolyte and catholyte) are flowed through the half-cells as the redox reaction occurs, often with an external pumping system. In this manner, the membrane in a flow cell battery operates in an aqueous electrolyte environment. In some applications, an iron-ion containing aqueous hydrochloric acid solution may be used as the catholyte, while a chromium-ion containing aqueous hydrochloric acid solution may be used as the anolyte. In some applications, a mixture of chromium and iron containing solutions may be used on both sides of the redox cell. The use of mixed reactants eliminates the requirement for a highly-selective membrane since the composition of both half cells is identical in the discharged state.
In some redox flow batteries, certain metal impurities contained in the electrolyte solution can enhance side reactions at the negative electrode, which can result in the evolution of hydrogen gas that adversely affects the coulombic efficiency of the battery. While the use of high-purity raw materials such as high-grade iron chloride and high-grade chromium chloride can minimize such hydrogen gas-forming reactions, such materials are typically too expensive for use in redox batteries on a commercial scale.
Therefore, there exists a need to develop methods for preparing and purifying electrolyte solutions from inexpensive raw materials. In addition, there exists a need to develop methods of removing impurities from those electrolyte solutions, and, in particular, those impurities associated with the evolution of hydrogen gas and decreased efficiency of the flow cell battery.
Methods for the preparation and purification of electrolyte solution from raw materials are known. For example, one method involves preferential plating of nickel metal on an electrode of a flow battery using lead ions to provide the hydrogen overpotential and aid the process. However, considerable hydrogen evolution still occurs to impede the process. It is desirable to have a chemical method that avoids this problem and is amenable to scale up.